Orientational ordering of nanorods in diblock copolymers

ABSTRACT

Orientational ordering of rod-like nanoparticles in the lamellae phase of diblock copolymers has been considered theoretically using the model of a nanoparticle with two interaction centres. It has been shown that strongly anisotropic nanoparticles order spontaneously in the boundary region between the blocks where the orientational order is induced by the interface and by the interaction with monomer units in different blocks. The nematic order parameter possesses opposite signs in adjacent blocks which means that the nanorods are aligned parallel or perpendicular to the boundary between the blocks on different sides of their interface. Concentration and nematic order parameter profiles have been calculated numerically for different values of the nanoparticle length and compared with the results of recent computer simulations and with the results of the previous molecular theory based on nanoparticles of spherical shape.     

Features of structural relaxation in diblock copolymers

Time- and temperature-dependent structural relaxation (physical aging) of poly (styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymers was investigated by calorimetry. Our study reveals the interplay of the relaxation responses of the two components of the copolymer in an intermediate temperature regime. That is, when the testing temperature is closely below the glass transition temperatures of PS and PMMA, structural relaxation in these polymer phases takes place concurrently, the corresponding thermogram displays partially superposed dual endothermic peaks as a feature of physical aging in the diblock copolymers. The aging response for each component is identified from a curve fitting method and analyzed by the relaxation of enthalpy. Comparing with the homopolymer analogs, the PS and PMMA in diblock copolymers show enhanced aging rate.     

Calibration of chromatographic systems for quantitative prediction of chromatography of homopolymers

A method is proposed for obtaining from the experimental data the main parameters, which determine the chromatographic behavior of homopolymers: a dependence of the adsorption interaction parameter on the mobile phase composition, and the pore volume and pore size values. The interaction parameter can be determined by this method in both adsorption, critical, and SEC-type regimes of chromatography. The method is tested in ‘mathematical experiments’ with theoretical data that mimic experiments, and is applied to calibrate real chromatographic systems. With the obtained calibrations quantitative modeling of chromatograms of PEG samples on two typical ODS columns with different pore size in methanol/water and acetone/water mixed solvents is performed; the simulation results are compared with real chromatograms.     

A data base for polymer chromatography: Temperature dependence of interaction parameters in liquid adsorption chromatography

The temperature dependence of retention behaviour of polyethylene glycol (PEG) and its mono‐ and dimethyl ethers was studied on various RP columns in different mobile phases. The accessible volumes and the interaction parameters were determined from slope and intercept in a plot of the elution volumes of the oligomers of a polymer homologous series as a function of the difference of the elution volumes of consecutive oligomers. A quite different dependence of the interaction parameters was observed in the different mobile phases. While in methanol–water the interaction parameter decreases with increasing temperature, the opposite effect is observed in acetonitrile (ACN)–water. In acetone–water, the temperature dependence is almost negligible.     

Problems related to the optimum separation conditions in liquid adsorption chromatography

Summary Relationships derived from the thermodynamic formulation of TLC with a binary mobile phase are discussed. The adsorption equilibrium constant is determined from the linear form of a basic equation considering the adsorption process. This linear relationship is examined with help of TLC data obtained by using six different chromatographic systems. The adsorption equilibrium constant obtained from the TLC data is compared to the equilibrium constant determined from adsorption measurements. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Characterization of poly(2-oxazoline) homo- and copolymers by liquid chromatography at critical conditions

In this study liquid chromatography at critical conditions for poly(2-ethyl-2-oxazoline)s (PEtOx) has been performed for the first time in order to analyze functional PEtOx homopolymers and block copolymers. Besides the verification of the critical point of adsorption with two series of ester end group functionalized PEtOx homopolymers, to evaluate the effect of both the chain length dependence and the end group polarity, using a cyano column with a solvent combination of 2-propanol and water, also two-dimensional liquid chromatography (2D-LC) has been applied for a poly(2-oxazoline) block copolymer. The combined characterization techniques provided further information about the polymerization procedure with regard to the formation of side-products by separation of the block copolymer from the corresponding homopolymer impurities. In addition, hyphenation of LCCC with MALDI-TOF MS and ESI-Q-TOF tandem mass spectrometry verified the obtained results.     

Two-dimensional liquid chromatography/mass spectrometry/mass spectrometry separation of water-soluble metabolites

Off-line two-dimensional liquid chromatography with tandem mass spectrometry detection (2D-LC/MS-MS) was used to separate a set of metabolomic species. Water-soluble metabolites were extracted from Escherichia coli and Saccharomyces cerevisae cultures and were immediately analyzed using strong cation exchange (SCX)-hydrophilic interaction chromatography (HILIC). Metabolite mixtures are well-suited for multidimensional chromatography as the range of components varies widely with respect to polarity and chemical makeup. Some currently used methods employ two different separations for the detection of positively and negatively ionized metabolites by mass spectrometry. Here we developed a single set of chromatographic conditions for both ionization modes and were able to detect a total of 141 extracted metabolite species, with an overall peak capacity of ca. 2500. We show that a single two-dimensional separation method is sufficient and practical when a pair or more of unidimensional separations are used in metabolomics.     

Comparison of the adsorption mechanisms of pyridine in hydrophilic interaction chromatography and in reversed-phase aqueous liquid chromatography

The adsorption isotherms of pyridine were measured by frontal analysis (FA) on a column packed with shell particles of neat porous silica (Halo), using water–acetonitrile mixtures as the mobile phase at 295 K. The isotherm data were measured for pyridine concentrations covering a dynamic range of four millions. The degree of heterogeneity of the surface was characterized by the adsorption energy distribution (AED) function calculated from the raw adsorption data, using the expectation-maximization (EM) procedure. The results showed that two different retention mechanisms dominate in Per aqueous liquid chromatography (PALC) at low acetonitrile concentrations and in hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. In the PALC mode, the adsorption mechanism of pyridine on the silica surface is controlled by hydrophobic interactions that take place on very few and ultra-active adsorption sites, which might be pores on the irregular and rugose surface of the porous silica particles. The surface is seriously heterogeneous, with up to five distinct adsorption sites and five different energy peaks on the AED of the packing material. In contrast, in the HILIC mode, the adsorption behavior is quasi-homogeneous and pyridine retention is governed by its adsorption onto free silanol groups. For intermediate mobile phase compositions, the siloxane and the silanol groups are both significantly saturated with acetonitrile and water, respectively, causing a minimum of the retention factor of pyridine on the Halo column.     

Micellisation and gelation of diblock copolymers of ethylene oxide and propylene oxide in aqueous solution, the effect of P-block length

The aqueous solution properties of five diblock copolymers prepared by sequential anionic copolymerisation (i.e. E₁₀₂P₃₇, E₁₀₄P₅₂, E₉₂P₅₅, E₁₀₄P₆₀ and E₉₈P₇₃ where E denotes oxyethylene and P denotes oxypropylene) were studied across a wide range of concentration. The techniques used to study micellisation and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and eluent gel-permeation chromatography. The gelation of concentrated solutions was also investigated. As expected, the critical micelle concentration (CMC) was lowered and the association number of the micelles was increased by an increase in P-block length. In contrast, the critical gel concentration was unchanged, consistent with the constant E-block length leading to micelles with essentially identical E-block fringes. Comparison of the CMCs of the diblock copolymers with those of triblock E_mP_nE_m copolymers with the same P-block length shows the diblock copolymers to micellise more efficiently. A similar comparison of the CMCs of the diblock copolymers with those of E_mB_n copolymer (B denotes oxybutylene) shows the hydrophobicity of a P unit to be one-sixth that of a B unit. The possibility is explored of correlating the limiting association number of a spherical micelle with the hydrophobe block length of its constituent copolymer. Of the five copolymers, only dilute solutions of E₉₈P₇₃ were predominantly micellar at both room temperature and body temperature, and this copolymer must be a prime candidate in any consideration of the potential application of E_mP_n copolymers in the solubilisation and controlled release of drugs.     

Hydrophilic interaction liquid chromatography (HILIC) in proteomics

In proteomics, nanoflow multidimensional chromatography is now the gold standard for the separation of complex mixtures of peptides as generated by in-solution digestion of whole-cell lysates. Ideally, the different stationary phases used in multidimensional chromatography should provide orthogonal separation characteristics. For this reason, the combination of strong cation exchange chromatography (SCX) and reversed-phase (RP) chromatography is the most widely used combination for the separation of peptides. Here, we review the potential of hydrophilic interaction liquid chromatography (HILIC) as a separation tool in the multidimensional separation of peptides in proteomics applications. Recent work has revealed that HILIC may provide an excellent alternative to SCX, possessing several advantages in the area of separation power and targeted analysis of protein post-translational modifications. Figure Artistic impression of the HILIC separation mechanism     

Surface characterization of macroporous glycidyl methacrylate based copolymers by inverse gas chromatography

Two samples of macroporous crosslinked poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate), PGME, with different porosity parameters were synthesized by suspension copolymerization and modified by ring-opening reaction of the pendant epoxy groups with ethylene diamine, EDA. Inverse gas chromatography at infinite dilution was used for the determination of adsorption properties of PGME, and copolymer modified with ethylene diamine, PGME-en. Thermodynamic parameters of adsorption, dispersive components of the surface free energies, and the acid/base constants for the copolymer samples were calculated. The calculated dispersive surface energy values, , for PGME and PGME-en are comparable with the literature data for nonconductive polymers.     

Analysis of polystyrene-b-polyisoprene copolymers by coupling of liquid chromatography at critical conditions to NMR at critical conditions of polystyrene and polyisoprene

For the investigation of the molecular heterogeneity of polystyrene-b-polyisoprene block copolymers, a chromatographic separation method, namely liquid chromatography at critical conditions was developed. This method was coupled on-line with (1)H-NMR(where NMR stands for nuclear magnetic resonance) for the comprehensive analysis of the polystyrene-b-polyisoprene copolymers. The copolymers were synthesized by two different methods: sequential living anionic polymerization and coupling of living precursor blocks. While (1)H-NMR allows just for the analysis of the bulk chemical composition of the block copolymers, the coupling with liquid chromatography at critical conditions provides selective molar mass information on the polystyrene and polyisoprene blocks within the copolymers. The polyisoprene block molar mass is determined by operating at chromatographic conditions corresponding to the critical point of adsorption of polystyrene and size exclusion chromatography mode for polyisoprene. The molar mass of the polystyrene block is determined by operating at the critical conditions of polyisoprene. In addition to the molar mass of each block of the copolymers, the chemical composition distribution of the block copolymers was determined. By using the coupling of liquid chromatography at critical conditions to (1)H-NMR, one can also detect the homopolymers formed during synthesis. Finally the microstructure of the polyisoprene block in the copolymers was evaluated as a function of molar mass.     

Intermolecular interactions in liquid adsorption chromatography

Summary For the investigation of intermolecular interactions in adsorption from solution, which are the basis of selectivity in molecular liquid chromatography (LC), it is convenient to use the LC method itself. Using this method the Henry's constants, K₁, and other thermodynamic adsorption characteristics of hydrocarbons and of a series of polar substances on hydroxylated silica surface were determined from aqueous solutions. On the basis of the adsorption of hydrocarbons from water solutions the structure of the chemically modifying layers formed by different hydrocarbon groups on the silica surface is considered. The role of conformation ability of straight-chain bonded phases is demonstrated. Hydrocarbons are adsorbed on the hydroxylated silica surface more strongly from aqueous solutions than from solutions in saturated hydrocarbons and their retention increases with the increase in the number of carbon atoms in the molecule. The retention in LC is determined by the intermolecular interaction of the solute and solvent molecules with the adsorbent, as well by the contribution of the intermolecular interaction, between the solute and the solvent.The thermodynamic characteristics of adsorption of cymarin from water-ethanol solutions on hydroxylated silica gel and on silica gel surface modified by diphenylsilyl groups is compared. The solubility of silica gel modified by diphenylsilyl groups at different composition of water-ethanol eluent at different temperatures is investigated.Enlarged text of a paper presented at the Sixteenth International Symposium on Advances in Chromatography, Barcelona, Spain, September 28–October 1, 1981.     

Reduced sample recovery in liquid chromatography at critical adsorption point of high molar mass polystyrene

Low sample recovery may represent an important drawback in liquid chromatography at the critical adsorption point (LC-CAP) if the critical eluent is not carefully fitted to the system. So far, this problem was often overlooked and only few experimental examples can be found in literature. We showed that in the case of polystyrene (PS) in a tetrahydrofuran (THF)/n-hexane critical mixed eluent, PS with molar masses higher than 100 kg mol⁻¹ were not eluted from a tandem of two columns packed by bare silica gels with 30 nm and 100 nm pore size, respectively. The polymer trapped within the columns was well recovered after injection of a small volume of pure THF as demonstrated using 2D chromatography. We studied PS conformations by means of small angle neutron scattering and found that the THF/n-hexane critical eluent is in fact a theta solvent for PS. By replacing it by a CH₂Cl₂/n-hexane critical mixture, which is a good solvent for PS, the limits of reduced sample recovery was displaced towards far higher molar masses. Thus, thermodynamic quality of eluent - theta or good solvent - plays an important role on the phenomenon of sample recovery.     

Poly(styrene-b-2-vinylpyridine-1-oxide) diblock copolymers: 3. Microemulsions in water/propan-2-ol/toluene mixtures

Pseudo-ternary phase diagrams have been constructed for the three-component solvent system (toluene+water+propan-2-ol) containing diblock copolymers of poly(styrene-b-2-vinylpyridine-1-oxide). Microemulsions have been shown to form on the water-rich side of the phase diagram, in the region of the phase boundary without polymer. Dynamic light-scattering experiments have led to droplet size values in the region of 100 nm, with the size depending strongly on the propan-2-ol/water concentration, as well as the amount of solubilised toluene in the core. Viscometry experiments have been carried out to measure polymer aggregation numbers in the microemulsion droplets, and interfacial tension measurements have shown that in the absence of propan-2-ol (effectively a cosurfactant) the limiting value of the oil/water interfacial tension, even an saturation adsorption of the copolymer is 20 mNm^–1. However, addition of propan-2-ol reduces the interfacial tension to the very low values generall commensurate with microemulsion formation.     

High-temperature two-dimensional liquid chromatography of ethylene-vinylacetate copolymers

Temperature rising elution fractionation hyphenated to size exclusion chromatography (TREF × SEC) is a routine technique to determine the chemical heterogeneity of semicrystalline olefin copolymers. Its applicability is limited to well crystallizing samples. High-temperature two-dimensional liquid chromatography, HT 2D-LC, where the chromatographic separation by HPLC is hyphenated to SEC (HPLC × SEC) holds the promise to separate such materials irrespective of their crystallizability. A model blend consisting of ethylene-vinyl acetate (EVA) copolymers covering a broad range of chemical composition distribution including amorphous and semicrystalline copolymers and a polyethylene standard was separated by HT 2D-LC at 140 °C. Both axes of the contour plot, i.e. the compositional axis from the HPLC and the molar mass axis from the SEC separation were calibrated for the first time. Therefore, a new approach to determine the void and dwell volume of the developed HT 2D-LC instrument was applied. The results from the HT 2D-LC separation are compared to those from a cross-fractionation (TREF × SEC) experiment.     

Theory of chromatographic separation of linear and star-shaped binary block-copolymers

Equations for the distribution coefficient of heteroarm stars are derived by using a model of an ideal chain in a slit-like pore; these equations together with those previously reported for linear block-copolymers are applied to describe chromatography of such copolymers. According to the theory, the retention generally depends on molar mass, composition, and architecture (microstructure and topology) of copolymers, on pore size and on adsorption interaction of chain units A and B. Three special modes of chromatography are studied in detail. (i) If interactions for A and B are close to the critical point of adsorption (CPA), the retention practically does not depend on architecture, and high molar mass copolymers can be separated by composition. (ii) At SEC condition for B and strong adsorption for A, copolymers in principle can be separated by architecture; better separation is expected in wide pores. Retention of linear block-copolymers decreases with increasing of the number of blocks; for heteroarm stars the theory predicts retention decreasing as: AB > StarAAB > StarABB; StarAAAB > StarABBB > StarAABB; StarAAAAB > StarABBBB > StarAAABB > StarAABBB. (iii) At the CPA for B copolymers AB, BAB and heteroarm stars regardless molar mass of B, M(B), can be separated by M(A). The same is true for ABA and ABAB...A in narrow pores. While the retention of AB, BAB, Star AB...B and StarAAB...B is the same, copolymers AB, ABA and linear multiblock-copolymers can be separated, as well as symmetric and very asymmetric triblock-copolymers ABA.     

Separation of nonionic surfactants according to functionality by hydrophilic interaction chromatography and comprehensive two-dimensional liquid chromatography

It is shown, that amphiphilic polymers--such as polysorbates and fatty esters of polyethylene glycol can be separated by comprehensive two-dimensional liquid chromatography using a reversed phase column (under critical conditions for the polyoxyethylene chain) and a HILIC column, which may arranged in different order. The mobile phases in both dimensions can be 93-97 wt% acetone water. As the retention of higher esters on the reversed phase column is very strong, this column should be used as the first dimension. On the HILIC column all fractions elute within a reasonably short time (at a flow rate of 2.5 ml/min within 2 min). With a flow rate of 0.1 ml/min in the first dimension, a full separation can be achieved in 90 min.     

Two-dimensional liquid chromatography analysis of synthetic polymers using fast size exclusion chromatography at high column temperature

In recent years, two-dimensional liquid chromatography (2D-LC) has been used increasingly for the analysis of synthetic polymers. A 2D-LC analysis provides richer information than a single chromatography analysis at the cost of longer analysis time. The time required for a comprehensive 2D-LC analysis is essentially proportional to the analysis time of the second dimension separation. Many of 2D-LC analyses of synthetic polymers have employed size exclusion chromatography (SEC) for the second-dimension analysis due to the relatively short analysis time in addition to the wide use in the polymer analysis. Nonetheless, short SEC columns are often used for 2D-LC analyses to reduce the separation time, which inevitably deteriorates the resolution. In this study, we demonstrated that high temperature SEC can be employed as an efficient second-LC in the 2D-LC separation of synthetic polymers. By virtue of high temperature operation (low solvent viscosity and high diffusivity of the polymer molecules), a normal length SEC column can be used at high flow rate with little loss in resolution.